1. Field of the Invention
The invention relates to a process for the production of a color image, in particular for the color testing of originals for multicolor printing. The invention further relates to a layer material that is suitable for carrying out the process and to a color image, in particular a multicolor image, which is obtained by the process.
2. Description of Related Art
In multicolor printing, a color test generally is carried out to permit the correction of the color separations used for the exposure of printing plates. The color proof must be a true image of the desired halftone or line image and should reproduce the tonal value of the colors faithfully and as far as possible identically to the subsequent printed image. The visual color test should indicate errors in the original, make it possible to recognize the best color reproduction to be expected during printing on the press, reproduce the exact gradation of all hues and show whether the gray tints are neutral. They should furthermore indicate the necessity of reducing one of the colors and/or provide information regarding any change to the photographic transparency prior to production of the printing plates.
In this color test, images typically are produced in succession in primary colors, according to the primary colors of the multicolor print, on an image-receiving material and then transferred to this material so that the in-register superposed images in primary colors give a faithful multicolor image.
The primary color images can be produced, inter alia, by exposing and washing out a finished photosensitive layer, by exposing and peeling apart the support film and cover film of a layer material having a photosensitive layer in between, the exposed parts of the layer adhering to one film and the unexposed parts to the other film, (peel-apart process), or by treating a developed colorless image with a toner or with ink. In general, diazo compounds or photopolymerizable mixtures are used as photosensitive components.
For the production of a multicolor image from individual images in primary colors, it has proven particularly suitable to laminate the photosensitive layers for the individual colors in succession with an image-receiving material, to expose them there and to develop them to give the image in primary color. This procedure is preferred to the possibility of producing each image in primary color separately and transferring the images in primary colors in succession to the image-receiving material. The latter case requires lamination in register, which is very much more difficult to effect in terms of apparatus than exposure in register.
U.S. Pate. Nos. 4,260,673, 4,650,738, 4,656,114 and 4,659,642, and EP-A 447 829 describe photosensitive materials based on diazo compounds for the production of color proofing films, which are processed in a manner such that the unexposed photosensitive layer is first laminated with the image-receiving material and then exposed in register and developed by washing out. U.S. Pat. Nos. 4,895,787 and 5,049,476 describe color proofing methods in which photosensitive color layers are likewise transferred to an image-receiving material, exposed there and developed. These materials are based on photopolymerizable mixtures and are developed by the "peel-apart" technique.
All these materials with which multicolor images are produced from images in primary colors by combination in register contain release layers and/or adhesion-promoting layers which remain between the individual images in primary colors in the finished multicolor image. However, these colorless intermediate layers cause optical magnification of the individual image elements, i.e. of the halftone dots in halftone images, which generally is referred to as optical dot enlargement. This optical dot enlargement is entirely desirable and necessary in order to approximately simulate the dot enlargement which, in offset printing, may be in the range of about 8-30% compared with the original. The dot enlargement in the color proof can be controlled within certain limits by increasing or reducing the layer weight of the release and adhesion-promoting layers. Thus, for example, U.S. Pat. No. 4,262,071 describes the introduction of a spacer layer for increasing the dot enlargement.
The adjustment of dot enlargements in the higher range can be readily effected by the known methods. It is substantially more difficult to adjust enlargements in the range from 8 to 20%, i.e. to reduce the enlargement occurring in the uncorrected color image. EP-A 305 599 describes the reduction by the use of a support material which has a coating with TiO.sub.2. Another method of reduction is described in U.S. Pat. No. 4,734,356. There, positive-working color proofing films which are based on photopolymerizable mixtures containing optical brighteners are rendered visible by treatment with a toner. U.S. Pat. No. 4,889,787 describes photosensitive color proofing films which contain very thin adhesion-promoting layers (1.0 to 3.2 g/m.sup.2) and thus give very small dot enlargements. However, the processing requires relatively high lamination temperatures and low lamination speeds.
EP-A 420 675 describes a color proofing method in which TiO.sub.2 is added to the adhesion-promoting layer of the color proofing film, which is the first to be laminated with the receiving material. This adhesion-promoting layer is substantially thicker than those in the other films.
This method requires that the image is always built up starting with the same primary color, i.e. the color film which carries the thick pigmented adhesion-promoting layer. Furthermore, only a slight reduction in the dot enlargement is achieved.
Thus, there exists a need to provide a process for the production of color images, in particular multicolor images, in which the dot enlargement in the proof can be reduced to the desired extent and in which image defects caused by dust particles on the surface of the layer to be transferred can be avoided during processing by the peel-apart method.